A concentrator or hub of a computer Local Area Network (LAN) has a plurality of individual cards or modules. The cards or modules in a concentrator can provide connections to individual workstations, connections to other concentrators or hubs, management functions, control functions and power supplies. The modules or cards are connected by a backplane. Data signals and power are conveyed across the backplane, and each of the cards is connectable to the backplane.
Data is usually transferred across the backplane in parallel. There are a plurality of data paths, with at least one data path for each bit in a word. In this way an entire word can be transmitted across the backplane in a single clock cycle. In ATM networks, data is transferred in cells, where each cell has a plurality of words or bytes. Each word or byte contains a plurality of bits, usually 8. These 8 bits are usually transmitted in parallel over the backplane. Sometimes even the bits of two or four words are transmitted in parallel across the backplane in a single clock cycle. Correspondingly, there must be a line for each of the bits transmitted in parallel.
In an ATM cell, the bits of one word are transmitted or stored in parallel to or in the card, the individual words follow in series.
The individual bits of a data word can also be transmitted serially across a serial line. Some serial lines are AC coupled in order to keep opposite ends of the serial line electrically independent. This requires that the data signal alternate frequently between high and low bits, and that the number of high and low bits be maintained substantially equal.
A known method for transmitting data across an AC coupled serial line, involves the known 8B/10B encoding/decoding mapping copyrighted by IBM in 1983. This 8B/10B encoding or mapping actually takes 8 data bits and one control bit, and converts this into a 10 bit output word. The 10 bit output word is chosen to have sufficient variation in high and low bits to properly convey the data across the AC coupling. The control bit is used to indicate to the 8B/10B encoding when the 8 bit word being received is not a data word, but is instead a word indicating a boundary between words or some other control function.
The 8B/10B encoding also keeps track of the number of high and low bits that have been transmitted across the serial path. When the 8B/10B encoding receives a 9 bit word indicating the control word for a boundary or comma between data words, or a data word itself, the 8B/10B encoding can transmit either a first or second version of the corresponding 10 bit control word. One version (positive) has more high bits than low bits, and the other version (negative) has more low bits than high bits. The 8B/10B encoding selectively uses the first and second versions of the control comma word to maintain an even balance of high and low bits transmitted over the serial path.
Serializing and deserializing circuits are known which receive the 10 bit words from the 8B/10B encoder, and convert these 10 bits into a serial data stream. The deserializer circuit receives the serial data stream and converts the serial data stream back into 10 bit parallel words. Such a serializer and deserializer circuitry is available from the manufacturer AMCC, 6195 Lusk Blvd., San Diego, Calif. 92121, as part number S2052. The deserializer circuit or chip recognizes only one version of the control comma word and uses this one version to synchronize the deserialization process to properly combine the received serial bits into the corresponding parallel data word. The one version of the control comma word indicates where in the serial data stream the data word starts. Not only does this chip only recognize the one version of the control comma word, but this chip expects this control comma word to only appear in certain periodic locations. This deserializer chip or circuitry has been specifically designed for an application where the incoming data stream causes sufficient use of the one version of the control comma word so that the deserializer circuit can maintain synchronous operation. The application also is such that the control comma word is only used at the periodic locations.
In ATM networks, information is sent in the form of individual data cells which contain a plurality of data words. When an ATM data cell is received by a card in a hub or concentrator, it can be processed by an Atlanta chip set interface, namely the ABM-to-ASX interface which has its own corresponding ATM interface format. This ATM interface format consists of 8 bits of data, one bit of parity and one bit of Start-of-Cell (SOC) indication. Once started, data will be sent across this interface in 68-byte portions (53 bytes of ATM cell, plus 15 bytes of overhead), without any gaps between the cells as shown in FIG. 1. This interface format is identical to the format of the ABM (Lucent Technologies LUC4AB01) and ASX (Lucent Technologies LUC4AS01).